a) Field of the Invention
This invention relates to a fluid valve assembly for regulating the flow rate of a fluid or for a like purpose. Between a valve member and a drive unit for driving the valve member, the fluid valve assembly is provided with a seal structure that prevents entrance of the fluid from a flow passage, in which the valve member is arranged, to the side of the drive unit. In particular, the present invention is concerned with such a fluid valve assembly suitable for use in a valve unit of an exhaust gas recirculation system for recirculating to an intake side an exhaust gas emitted from an internal combustion engine such as an automotive vehicle.
b) Description of the Related Art
A fluid valve assembly which functions to regulate the flow rate of a fluid is provided with a drive unit for driving a valve member. Conventional fluid valve assemblies include those provided with drive units for driving their valve members, respectively. In each of such conventional fluid valve assemblies, there is a slide contact area between the valve member and a valve seat supporting the valve member thereon and defining an orifice of a flow passage therethrough. With the slide contact area serving as a boundary, its drive unit is arranged on an outer side of the flow passage. In some applications, it is desired to protect the drive unit from the fluid or from impurities contained in the fluid. Such fluid valve assemblies include, for example, exhaust gas recirculation control valves (hereinafter simply called “EGR (Exhaust Gas Recirculation) valves” for the sake of brevity).
An EGR valve is used in an exhaust gas recirculation system for an internal combustion engine such as an automotive vehicle, and is arranged to inhibit the occurrence of harmful components in an exhaust gas by recirculating an appropriate amount of the exhaust gas to an intake side in accordance with a state of running of an automotive vehicle or the like.
This EGR valve is effective not only for a gasoline engine but also for a diesel engine to lower NOx (nitrogen oxides) in an exhaust gas. In the case of a diesel engine, however, the setting of the amount of an exhaust gas, which is to be recirculated (hereinafter called “the amount of the EGR gas”), at an excessively large value induces an increase in the content of PM (particulate matter) in the exhaust gas, while the setting of the amount of the EGR gas at an unduly small value is unable to sufficiently lower the content of NOx in the exhaust gas.
It is, therefore, necessary to perform an accurate control on the amount of an EGR gas (hereinafter called “EGR flow rate control”). This EGR flow rate control is generally performed by adjusting the opening of an EGR valve. As EGR valves, motor-driven EGR valves have been developed in recent years with a view to making improvements in the accuracy of control although pneumatic, pressure differential EGR valves making use of a positive pressure or a negative pressure are still in common use.
A motor-driven EGR valve is high in the accuracy of control, and has made it possible to properly control the EGR flow rate. Nonetheless, the entrance of impurities such as SOx, unburned fuel components and soot to the interior of the motor develops inconveniences such as corrosion and sticking motor, and in the worst case, involves a potential problem of a stall of the motor.
Described specifically, an EGR valve is arranged in an exhaust gas passage so that a poppet valve, butterfly valve or the like, which constitutes the EGR valve, is exposed to an exhaust gas. A motor which serves to drive the EGR valve is connected to such a poppet or butterfly valve or the like, and therefore, is in an environment where the exhaust gas can easily enter the interior of the motor. In particular, the exhaust gas is higher in pressure than the atmospheric pressure and is also high in temperature, so that the exhaust gas tends to enter the interior of the motor.
There is, accordingly, a need for a structure that prevents the exhaust gas from entering the interior of the motor.
As a conventional structure for preventing the entrance of an exhaust gas or the like to the side of a motor in a motor-driven EGR valve, there is, for example, a structure such as that illustrated in FIG. 5.
As depicted in FIG. 5, this EGR valve is provided with an exhaust gas passage 11 as a part of an exhaust gas recirculation passage, a valve element 12 interposed as a valve member in the exhaust gas passage 11 at an intermediate location thereof, an electric motor 13 adapted as a drive unit to drive the valve element 12 in an axial direction via a valve shaft 14 of the valve element 12, a bearing 15 supporting thereon the valve shaft 14 of the valve element 12, and a casing 16 with the bearing 15 and valve shaft 14 accommodated therein. When the electric motor 13 is rotated, the valve element 12 is driven in the axial direction via a translation mechanism (not shown), which translates the rotation of the electric motor 13 to an axial motion, so that the exhaust gas passage 11 can be opened or closed or its opening can be adjusted.
Between an inner circumferential wall of the casing 16 and an outer circumferential wall of the valve shaft 14, a resin-made seal diaphragm 17 is interposed. This seal diaphragm 17 is in a substantially funnel-shaped form. Its outer circumferential portion is secured in a gas-tight fashion on the inner circumferential wall of the casing 16, while its inner circumferential portion is maintained in sliding contact with the outer circumferential wall of the valve shaft 14. Under a condition that the pressure of an EGR gas is higher than the atmospheric pressure, the seal diaphragm 17 is caused to expand like a kite by the pressure of the EGR gas as shown in FIG. 5 so that its sealing performance is increased at a slide contact area between its inner circumferential portion and the outer circumferential wall of the valve shaft 14.
JP-A-5-187328, on the other hand, discloses a technique that provides an EGR valve with an impurities entrance blocking means, although the EGR valve is not of the motor-driven type but is of the pressure differential type.
As illustrated in FIG. 6, this EGR valve is provided with an exhaust gas passage 101, a valve element 102 interposed in the exhaust gas passage 101 at an intermediate location thereof, a diaphragm 103 to which the valve element 102 is connected, and a negative pressure chamber 104 for driving the valve element 102 in cooperation with the diaphragm 103. By depressurizing the negative pressure chamber 104, the valve element 102 is caused to move upward as viewed in FIG. 6, whereby a pintle 112 of the valve element 102 is separated from a valve seat 105 to open the exhaust gas passage 101. By releasing the reduced pressure in the negative pressure chamber 104, on the other hand, the valve element 102 is caused to move downwardly by a return spring 106 as viewed in FIG. 6 so that the pintle 112 is brought into close contact with the valve seat 105 to close the exhaust gas passage 101.
A valve shaft 107 of the valve element 102, which is a movable member, is provided with a shield plate 108, and an impurities blocking member 110 is arranged between the valve shaft 107 and a guide member 109 through which the valve shaft 107 extends and slidingly reciprocates up and down.
Opposing the guide member 109, the shield plate 108 is fixedly secured on an approximately central part of the valve shaft 107 as viewed in the longitudinal direction. This shield plate 108 has an outer diameter dimensioned slightly smaller than an inner diameter of a cavity 111, and can prevent an exhaust gas, which is passing through the exhaust gas passage 101, from flowing to the side of the guide member 109 and can also reduce a radiation of heat from the exhaust gas toward the side of the guide member 109.
The impurities blocking member 110 has approximately U-shaped configurations as viewed in cross-section, and its inner and outer surfaces are formed with corrugations such that it is provided with high elasticity and is facilitated to undergo flexible deformations. As the material of the impurities blocking member 110, a flexible, fluorinated resin is used for its excellent heat resistance and flexibility and its high abrasion resistance. The impurities blocking member 110 is provided at an upper circumferential edge thereof with a flange portion 110a molded integrally with the impurities blocking member 110, and at a lower end portion thereof with an opening 110c. The impurities blocking member 110 is disposed, with its flange portion 110b being fitted in an annular groove 111a formed adjacent the cavity 111 such that its upper surface is maintained in close contact with the guide member 109, and with its lower end portion 110b being positioned close to the shield plate 108 and its opening 110c being fixedly secured on the outer circumferential wall of the valve shaft 107.
As readily appreciated from the foregoing description, the impurities blocking member 110 is arranged between the guide member 109 and the shield plate 108 and, even when the valve element 102 is in its open position, shields, in other words, covers the guide member 109, including a slide contact area between an axial bore 109a and the valve shaft 107, in a shielded state to maintain gas tightness, so that soot and the like in the exhaust gas are prevented from entering the side of the guide member 109.
However, the above-described conventional art is accompanied with problems as will be described next.
In the case of the first conventional art depicted in FIG. 5, there is the slide contact area between the inner circumferential portion of the seal diaphragm 17 and the outer circumferential wall of the valve shaft 14. As the use of the EGR valve goes on, the seal diaphragm 17 hence wears out at the slide contact area so that the sealing performance is progressively lowered.
As a consequence, the exhaust gas which has entered through the slide contact area between the valve shaft 14 and the bearing 15 (see arrows A1,A2) is allowed to enter the side of the electric motor 13 through the slide contact area between the inner circumferential wall of the seal diaphragm 17 and the outer circumferential wall of the valve shaft 14 (see arrows A3). As the EGR valve is used for a longer time, the abrasion wear of the seal diaphragm 17 increases, eventually resulting in a potential problem that the seal diaphragm 17 may fail to exhibit its sealing function and the motor 13 may be damaged accordingly. Even when an abrasion-resistant material such as polytetrafluoroethylene, for example, “TEFLON” (registered trademark) is used for the seal diaphragm 17, it is still difficult to inhibit the reduction in sealing performance.
In the case of the second prior art illustrated in FIG. 6, on the other hand, such a slide contact area is included at neither the inner circumference nor the outer circumference of the impurities blocking member 110. Taking a look at the inner circumference of the impurities blocking member 110, however, the lower end portion 110b is fixedly secured only at an edge portion of the opening 110c on the outer circumferential wall of the valve shaft 107. There is, accordingly, a potential problem that the inner circumference of the impurities blocking member 110 may separate from the outer circumferential wall of the valve shaft 107 by the high-pressure, high-temperature EGR gas. Once such separation takes place, the impurities blocking member 110 can no longer exhibit its sealing function, leading to a potential problem that the motor may be damaged.
These problems are not limited to EGR valves, but may occur on any of various valves insofar as they include in the proximity of a movable member a drive means which should be protected from a fluid or from impurities or the like in a fluid (a means corresponding to an EGR valve drive motor).